This invention relates to a process and system for producing purified water utilizing reverse osmosis. More particularly, this invention relates to a process and system for purifying water by reverse osmosis followed by deionization of waste water from the reverse osmosis step in a manner which conserves water.
At the present time a reverse osmosis step is used to remove impurities, primarily microorganisms, pyrogens and ionic material from water by utilizing a reverse osmosis membrane. Usually, reverse osmosis is conducted by contacting the incoming water tangentially to the membrane so that the waste water comprising the retentate washes the membrane to reduce accumulation of impurities retained on the membrane during use. As sparingly soluble salts are concentrated in the concentrate (waste) stream, their concentrations must be kept low so as to prevent precipitation of the least soluble species thereby to maintain the membrane surface free of colloidal and particulate matter. The fraction of the total water which comprises the retentate varies depending on the quality of feed water, desired product quality and the type of membrane material employed in the reverse osmosis unit. In a common mode of single stage operation, between about 80 and 90% of the incoming water to the reverse osmosis step washes the upstream surface of the reverse osmosis membrane and is removed as retentate or waste. The remaining portion of the incoming water passes through the reverse osmosis membrane as purified water. This mode of operation, wherein a large percentage of the water constitutes reject water previously has been thought to be necessary in order to prevent premature plugging of the reverse osmosis membrane.
At the present time, 80 to 90% loss of water is considered unacceptable particularly in geographical areas where water is increasingly in short supply. A process which utilizes a reverse osmosis step is disclosed in U.S. Pat. No. 5,156,739 and which directs a large proportion of feed water to waste. Accordingly, it would be desirable to provide a means for effecting reverse osmosis treatment of water while avoiding high losses of waste water.